a) Field of the Invention
This invention relates to an attenuator for attenuating a microwave signal, and particularly to a variable attenuator which can vary attenuation by means of a control signal.
b) Description of the Prior Art
A microwave attenuator may be implemented as a fixed attenuator or a variable attenuator. A typical example of the variable attenuator is a reflection attenuator. The reflection attenuator can consist of a 4-port coupler, PIN diodes, and dummy loads.
FIG. 9 shows the construction of a reflection attenuator according to one prior art. This prior art uses a 4-port coupler (3) which has four ports (4 to 7). The port 4 and the port 5 are in radio frequency coupling, and a microwave signal inputted from the port 4 appears at the port 5 in an in-phase half amplitude. Similarly, the port 6 and the port 7 are in radio frequency coupling, and a microwave signal inputted from the port 6 appears at the port 7 in an in-phase half amplitude. The port 4 is connected to the port 6 by a line, and a microwave signal which is inputted in the port 4 appears at the port 6 in 90.degree.--delayed half amplitude. And, the port 5 is connected to the port 7 by a line, and a microwave signal which is inputted in the port 5 appears at the port 7 in a 90.degree.--delayed phase half amplitude. These relations constitute even when input and output are exchanged. Specifically, the 4-port coupler 3 is a quadrature hybrid coupler or a 3-dB range coupler, and the ports 4 to 7 are an input port, an in-phase output port, a 90.degree.--delayed phase output port and a 90.degree.--delayed phase combination output port, respectively.
In FIG. 9, the port 4 is connected to a signal input terminal 1, and the port 7 to a signal output terminal 2, respectively. A microwave signal which is inputted in the signal input terminal 1 appears at the port 5 in an in-phase half amplitude and at the port 6 in a 90.degree.--delayed phase half amplitude. The port 5 is connected to a termination circuit in which a transmission line 11a, a PIN diode 12a, and a dummy load 13a are connected in series in this order. Similarly, the port 6 is connected to a termination circuit in which a transmission line 11b, a PIN diode 12b, and a dummy load 13b are connected in series.
The transmission lines 11a and 11b have an impedance converting function. In this case, a characteristic impedance of the 4-port coupler 3 after the impedance conversion made by the transmission lines 11a and 11b is assumed to be Z0 as the matched impedance of the transmission line. Since the PIN diodes 12a and 12b function as variable radio frequency resistive elements, when synthesized resistance Z1a of the dummy load 13a and the PIN diode 12a is equal to Z0, the port 5 falls under a reflectionless termination state. Similarly, when synthesized resistance Z1b of the dummy load 13b and the PIN diode 12b is equal to Z0, the port 6 falls under a reflectionless termination state.
As described above, the signal which appears at the port 5 appears at the port 7 in a 90.degree.--delayed phase half amplitude, and the signal which appears at the port 6 appears at the port 7 in an in-phase half amplitude. Therefore, the signal appearing at the port 7 is a combined signal of a reflection signal by the termination circuit connected to the port 5 among the signals inputted from the port 4, and a reflection signal by the termination circuit connected to the port 6 among the signals inputted from the port 4. These reflection signals are in-phase with each other because they are opposite relative to the signal inputted in the port 4. Therefore, the combination at the port 7 is an in-phase combination. When both Z1a and Z1b are equal to Z0, both the ports 5 and 6 are subjected to the reflectionless termination, and no reflection signal appears at the port 7 (a state of infinite attenuation). On the other hand, when both Z1a and Z1b are substantially infinite, both the ports 5 and 6 fall under a total reflection state, and the maximum reflection signal appears at the port 7 (a state of minimum attenuation).
Therefore, if Z1a and Z1b could be varied successively in the range of Z0 to infinity, the attenuation of the signal outputted from the signal output terminal 2 with respect to the signal inputted from the signal input terminal 1 can be ideally controlled successively in the range of 0 to infinity. As a means therefor, the prior art provides the PIN diodes 12a and 12b and a bias control circuit.
As shown in FIG. 9, the signal input terminal 1 and the signal output terminal 2 have bias terminals 10a and 10b connected, respectively. Since the coupling of the ports 4 and 5 prevents a d.c. voltage, the d.c. voltage which is applied to the bias terminal 10a is applied to the anode of the PIN diode 12b. Similarly, since the coupling of the ports 5 and 7 prevents a d.c. voltage, the d.c. voltage which is applied to the bias terminal 10b is added to the anode of the PIN diode 12a. Accordingly, the resistance values of the PIN diodes 12a and 12b are determined by the d.c. voltage (bias voltage) which is applied to the terminal 10b or 10a, respectively. In other words, the successive variation of the bias voltage which is applied to the terminals 10a and 10b can successively vary Z1a and Z1b in the range of Z0 to infinity. Thus, the attenuation can be successively controlled in the range of 0 to infinity.
Furthermore, quarter wavelength lines 8a and 8b are respectively connected between the signal input terminal 1 and the bias terminal 10a and between the signal output terminal 2 and the bias terminal 10b. To the bias terminals 10a and 10b, quarter wavelength open stubs 9a and 9b which are quarter wavelength lines with one end open are connected, respectively. When the quarter wavelength open stubs 9a and 9b are observed from the bias terminals 10a and 10b, impedance thereof becomes 0. When this impedance is observed from the signal input terminal 1 or the signal output terminal 2, it is seen to be infinite because of the quarter wavelength lines 8a and 8b which are present between the signal input terminal 1 and the bias terminal 10a and between the signal output terminal 2 and between the bias terminal 10b.
Therefore, regardless of the connection of the bias terminals 10a and 10b to the signal input terminal 1 and the signal output terminal 2, respectively, for a signal which has a relatively high frequency and therefore a relatively short wavelength of the same length as that of an electrical length of each quarter wavelength line or open stub, the bias terminals 10a and 10b fall in a state not visibly existing. Thus, the quarter wavelength lines 8a and 8b function in the same way as an inductance in a Radio-frequency circuit, and the quarter wavelength open stubs 9a and 9b function in the same way as a capacitance, so that a circuit consisting of the quarter wavelength line 8a and the quarter wavelength open stub 9a and a circuit consisting of the quarter wavelength line 8b and the quarter wavelength open stub 9b can be understood by the analogy with a resonance circuit in a Radio-frequency circuit. These circuits are called RF chokes.
For accurate functioning of a variable attenuator having the circuit configuration as shown in FIG. 9, the resistance values of the PIN diodes 12a and 12b must be balanced accurately. Therefore, bias voltages which are added to the PIN diodes 12a and 12b must be also balanced accurately. But, it is difficult to secure such a balance in the construction as the bias voltages are separately applied to the PIN diodes 12a and 12b as in FIG. 9. Furthermore, the PIN diodes 12a and 12b generally have a stray capacity or a stray inductance.
Because the demands for securing balanced bias voltages of the PIN diodes 12a and 12b cannot be fully met, the attenuation cannot be fully and accurately controlled heretofore. Therefore, the variable attenuator can not avoid suffering from poor I/O characteristics, reflection characteristics, and frequency characteristics. And, attenuation characteristics also deteriorate when affected by a stray capacity or a stray inductance.